Because of its clean burning qualities and convenience, natural gas has become widely used in recent years. Many sources of natural gas are located in remote areas, great distances from any commercial markets for the gas. Sometimes a pipeline is available for transporting produced natural gas to a commercial market. When pipeline transportation is not feasible, produced natural gas is often processed into liquefied natural gas (which is called "LNG") for transport to market.
It has been recently proposed to transport natural gas at temperatures above -112.degree. C. (-170.degree. F.) and at pressures sufficient for the liquid to be at or below its bubble point. For most natural gas compositions, the pressure of the natural gas at temperatures above -112.degree. C. will be between about 1,380 kPa (200 psia) and about 3,500 kPa (500 psia). This pressurized liquid natural gas is referred to as PLNG to distinguish it from LNG, which is transported at near atmospheric pressure and at a temperature of about -160.degree. C.
If PLNG is unloaded from a container by pumping the PLNG out and allowing the container pressure to decrease, the decompression of the PLNG can lower the temperature in the container below the permitted design temperature for the container. If the pressure in the container is maintained as the PLNG is removed to avoid such temperature reduction, the vapor remaining in the container will contain a significant volume of the container's original cargo. Depending upon the pressure and temperature of storage and the composition of the PLNG, the vapors may constitute from about 10 to 20 percent of the mass of PLNG in the container before the liquid was removed. It is desirable to remove as much of this gas as is economically possible while keeping the container at approximately the same temperature as the PLNG before unloading.